This study involves the characterization of the primate polyomaviruses BK, JC, SV40, STMV, and SA12. Each of these polyomaviruses is oncogenic in hamsters and has been demonstrated to be capable of transforming certain rodent cells in tissue culture. BK and JC viruses are human polyomaviruses first described as isolates from human beings in 1971. While BK virus has not been clearly associated with any human disease, JC virus has been repeatedly isolated from the diseased brains of patients with progressive multifocal leukoencephalopathy (PML). The purpose of this work is to better characterize the human polyomaviruses BK and JC both in terms of their molecular characteristics as well as their biological properties. An understanding of how they interact with human cells will provide a better understanding of what pathogenic roles they may play in humans and what role, if any, they may have in human malignancy.